From the past, a number of the methods for shaping porous polypropylene have been proposed.
A general method for making a film involves an inflation film shaping method and a T-die sheet shaping method. In case a polyolefin is to be made porous according to such a method, a method wherein the polyolefin is impregnated with an incompatible organic or inorganic matter and then the surface between the polyolefin and the incompatible matter is destroyed by external force such as stretching or a method wherein a component incompatible with polyolefin is treated with an organic solvent, an acid or an alkali capable of dissolving the component is said to be general. Thus, it is rather rare that a single component polyolefin sheet obtained from a film-shaping apparatus is directly employed as an original film for preparing a porous film.
As an example of a porous film prepared from a single component film, there is mentioned a film prepared from an extrusion cold stretching method as disclosed in the Official gazette of Japanese Laid-open Patent Appln. No. Sho. 62-121737.
The cold stretching in this case is in principle uniaxial and therefore a balance in tensile strength of the film is extremely poor so that the tensile strength of the film in the direction perpendicular to the machine direction is as small as 20 MPa or less, thus limiting its industrial use limitative.
With a view to improving the strength in the direction perpendicular to the machine direction in this invention, Japanese Laid-open Patent Appln. No. Sho. 55-161830 discloses a solvent-stretching method for a microporous film.
This method enables stretching the film in transverse direction up to about 300% after stretching it in the machine direction so that a balance of the tensile strengths seems to be improved. However, tensile strengths themselves are extremely poor so that the strength of the stretched film is almost equal to unstretched film or is lower than the latter and at most as high as 30 MPa.
A high molecular weight polyolefin possesses excellent properties such as impact-resisting property, anti-abrasive property, chemicals-resisting properties and the like, as compared with a widely used polyolefin, and is now being studied for shaping it into films or sheets.
As the high molecular weight polyolefin is extremely higher in melt viscosity than the widely used polyolefin, however, the former cannot be shaped into films or sheets by the aid of an ordinary extruder unlike the widely used polyolefin.
In case a film or sheet excellent in tensile strength is to be obtained from the high molecular weight polyolefin, ordinarily the high molecular weight polyolefin is impregnated with a solvent or a plasticizer compatible therewith to decrease its melt viscosity before being shaped into a film or sheet by the aid of a conventional extruder and is then stretched or further treated for eliminating the solvent or plasticizer.
The solvent method and the plasticizer method employ a large amount of a solvent or plasticizer compatible with the high molecular weight polyolefin and a much larger amount of another solvent compatible with the above solvent or plasticizer for removing the first solvent or plasticizer to make the resultant film porous.